276 HOW WE FIGHT BACTERIAL DISEASES 



vaccines. The underlying principle in all is the same; certain 

 cells of the body are roused to activity, antibodies are formed, 

 and the invading organisms are destroyed and their toxins neu- 

 tralized. These conditions are brought about through the work 

 of the lysins, precipitins, agglutinins, opsonins, and phagocytes 

 already mentioned in Chapter XVI. 



Other vaccines are made and used successfully against boils, 

 still another against paratyphoid, and still others for plague and 

 cholera. When tests show sensitiveness to certain pollens, serums 

 are made from them and a certain amount of immunity from hay 

 fever is thus received. But we are just at the beginning of dis- 

 coveries along this line and it will no doubt be the work of the 

 physician of the future to perfect many more ways of producing 

 immunity against protein poisons and germ disease. 



Summary. — This chapter has shown us that disease germs take 

 a certain time to grow in the body before the effects are seen. 

 We cannot tell whether a person has a disease until after this 

 incubation period is over ; hence the necessity of quarantine. 



Immunity or protection against disease may be both natural 

 and acquired. The latter may be passive, through antitoxins 

 introduced into the body; or active. For active immunity, the 

 body works up its own resistance following the introduction of 

 (a) living germs, usually attenuated ; (b) dead germs ; or (c) ex- 

 tracts containing poisons made by the germs. Immunity is thus 

 obtained against a number of diseases, notably diphtheria, small- 

 pox, typhoid, and tetanus. Through these and other means it is 

 estimated that medical science has added twelve years to the 

 average life of man since the time of the Civil War. 



Problem Questions 



1. Why is quarantine a necessary precaution? 



2. Can a person have the germs of a disease in the body and still not 

 show symptoms of the disease ? How might such a person be a danger to 

 others? 



3. What is immunity ? What types do we have? 



4. Why are some persons more likely to take a disease than others? 



5. Why do some people have a disease more severely than others? 



6. Why does travel bring increased likelihood of disease ? 



7. How do bacteria cause disease? 



